1. Field of the Invention
The present invention relates to a torque detecting apparatus for detecting a torque applied to a rotor and a manufacturing method of the torque detecting apparatus.
2. Description of Related Art
An example of a torque detecting apparatus is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-149062. The torque detecting apparatus in Japanese Patent Application Laid-Open No. 2003-149062 comprises: an input shaft and an output shaft which are connected with each other by a torsion bar so that torque is applied thereto; a permanent magnet mounted on a peripheral portion of the input shaft and a pair of annular rotary magnetic rings which are arranged around the periphery of the permanent magnet and rotates integrally with the output shaft; a detecting unit, which is arranged around the periphery of the rotary magnetic rings, for detecting a torque applied to the input shaft; and a detection circuit board, which is connected with the detecting unit, for converting a magnetic flux density detected by the detecting unit into a voltage signal. The detecting unit has: two static magnetic rings which are arranged apart from each other in the axial direction of the rotor, have magnetism collecting projections projected from a portion thereof and are supported by a supporting member; and a magnetism sensitive element arranged between the magnetism collecting projections of the respective static magnetic rings, and the magnetism sensitive element is connected with the detection circuit board by a conductor.
The rotary magnetic rings are designed to rotate relative to the permanent magnet to change the magnetic flux density between the permanent magnet and itself in one direction and the other direction. The static magnetic rings are designed to collect the magnetic flux from the rotary magnetic rings to the magnetism collecting projections. The magnetism sensitive element is constructed with a Hall element having an electrical characteristic (resistance) to be changed by the action of the magnetic field and is constructed to change a detection signal, i.e. a detection torque, according to a change in the magnetic flux density which is generated between the magnetism collecting projections by the relative rotation.
A torque detecting apparatus constructed as mentioned above is incorporated into an electric power steering apparatus for a vehicle. An electric power steering apparatus comprises: the rotor having the input shaft linked with a steering wheel; a housing which has a cylinder portion surrounding the rotor and a retainer on which the detection circuit board is mounted in the radial direction through an opening formed at the peripheral wall of the cylinder portion; an electric motor linked with the output shaft via a reduction mechanism; and a control unit, which is connected with a drive circuit of the electric motor and with the detection circuit board and is constructed with a microprocessor, and is constructed to detect with the detecting unit (magnetism sensitive element) a rotational torque which is applied to the input shaft when the steering wheel is steered on the basis of a torsion which arises at the torsion bar and to drive the electric motor in a controlled manner on the basis of the detected torque.
The torque detecting apparatus is installed into the housing by inserting the detecting unit having the magnetism sensitive element connected with one end of the conductor into the cylinder portion from the axial direction of the housing and retaining the detecting unit in the cylinder portion. Moreover, the detection circuit board is inserted into the opening of the retainer from the radial direction and mounted in the retainer, and the other end of the conductor is connected with the detection circuit board.
In a torque detecting apparatus constructed as disclosed in Japanese Patent Application Laid-Open No. 2003-149062, however, since the detecting unit and the detection circuit board are formed separately from each other and it is necessary to connect the detecting unit with the detection circuit board by a conductor, there are problems that the number of components is large and the number of steps of the installation operation is large. Moreover, since the torque detecting apparatus is installed into an electric power steering apparatus by incorporating the detecting unit and the detection circuit board from two directions which cross each other, i.e. by inserting the detecting unit from the axial direction of the housing and inserting the detection circuit board into the retainer from the radial direction of the housing, and, further, since the detecting unit and the detection circuit board are connected with each other by a conductor after installation of the detecting unit and the detection circuit board, the installability is law and a solution has been required.
Furthermore, since a permanent magnet is used, magnetic dust such as iron powder may enter the magnetic circuit causing a short circuit of the magnetic circuit and a deviation of a detection signal of the magnetism sensitive element, and a solution has been required. Moreover, since a space exists between the permanent magnet provided on the input shaft and the rotary magnetic rings provided on the output shaft to allow relative rotation, the run-out of the input shaft and the output shaft may cause a contact between the permanent magnet and the rotary magnetic rings and a short circuit of the magnetic circuit, and a solution has been also required for this point.